SMALL SCALE SEWAGE TREATMENT METHODS - Cesspoll, Septic Tanks and Imhoff's Tank

METHODS FOR SMALL SCALE SEWAGE TREATMENT

 

1) CESSPOOLS OR CESSPIT

• In many homes, human waste is emptied into underground Cesspools.
• Cesspool is an underground construction consisting of Concrete cylindrical rings with pores in the walls of the ring.

Cesspool

• Sewage water passes into the Soil through the bottom and pores of the Cesspool, while Solid waste accumulates on the bottom.
• Microorganisms, especially Anaerobic bacterial species, digest the Solid matter into Soluble products that enter the soil and enrich it.
• The amount of breakdown products exceeds, it forms Thick layers which need to be cleaned by using Strong acids.
• Dried bacterial preparation of Bacillus subtilis or Yeast cells or Enzymes should be added at intervals. These accelerate the decomposition of sludge deposited at the bottom of cesspool.
• The minimum size Cesspool for a habitable dwelling is 18000 litres and on average this Cesspool tank will require emptying once completely filled or every 20 - 25 days.

2) SEPTIC TANKS

• Septic tanks are mainly designed for On-site treatment of Domestic sewage.
• Septic tanks are small, rectangular chambers situated just below ground level where sewage is retained for 1 - 3 days.
• Septic tank is made of Brick work, Concrete, Fibre glass, PVC or Plastic.
• Septic tanks usually consist of 2 compartments. The First compartment is larger than the Second compartment.

Septic Tank

Longitudinal section of Septic tank

 Anaerobic digestion and Aerobic oxidation in Septic Tank

• The human excreta consist of 65 % Mineral matter (which do not undergoes any chemical changes in Septic tank) and 35 % of Organic matter (liquified or gasified in Septic tank).
• Bacteria are by a wide margin the most numerous microbes in Septic systems.
• A typical Septic system accomplishes treatment in a Two phase process.

a)    First phase (Anaerobic digestion)

ü  Solids introduced to the Septic tank react with bacteria and broken into simpler compounds. This process is called Anaerobic digestion.

ü  Heavier matter (Solids) settle to the bottom of the tank where they are digested Anaerobically and the Lighter matter (Grease and Fats) forms the layer called Scum on the top surface. The Scum at the surface and helps maintain Anaerobic conditions.

ü  Anaerobic digestion which occurs in Septic tanks represents an incomplete digestion. Methane, Hydrogen sulfide, and Sulfur dioxide gases are produced, as well as a Sludge of high molecular weight Hydrocarbons.

ü  Sometimes, the Sludge accumulates at the bottom of the tank that needs regular Desludging.

ü  Biogas is produced in a Septic tank as Sludge decomposes (Desludging) and Gas rises to the surface as bubbles.

b)    Second phase (Aerobic oxidation)

ü  The liquid which passes out of the outlet pipe from time to time is called Effluent.

ü  The effluent contains numerous bacteria, cyst, helminthic ova, organic matter and allowed to percolate into the sub-soil. There are millions of aerobic bacteria in the upper layer of soil and it digests the Organic matter present in effluent. Thus, the organic matters are oxidized into stable end products like Nitrates, Carbon-di-oxide and Water.

Performance of Septic tank in Pollution reduction

·       COD reduction – 25 to 50 %

·       BOD reduction – 40 %

·       Suspended solids reduction – 65 %

·       Microbial reduction - Effluent still contains pathogenic Bacteria, Cysts and Worm eggs.

Advantages of Septic tank

• Low cost
• Low land space required due to underground construction
• No electricity requirement
• Low operational and maintenance requirement
• Construction materials locally available
• No real problems with flies or odours if used correctly

Disadvantages of Septic tank

• Low effluent quality
• Possible for microbial contamination
• Regular desludging must be ensured
• Effluent and sludge require further treatment and appropriate discharge
• Manual cleaning of the tank is highly hazardous and an inhumane task, while mechanical cleansing (vacuum trucks) requires sophisticated instruments.

3) IMHOFF’S TANK

• The Imhoff tank (also known as Emscherbrunnen or Emscher Tank), which works similar to a communal Septic tank.
• The Imhoff tank obtained its name from its inventor, Dr. Karl Imhoff of Germany. The technology was developed in the Emscher District of Germany and patented in 1906 by Dr. Imhoff. 

Imhoff’s Tank

• The Imhoff tank is a compact treatment unit usually made of concrete and build in-ground.
• Imhoff tank consists of a V-shaped settling chamber with a residence time of 1 to 3 hours.
• The Imhoff Tank consists of (a) Upper section known as the Sedimentation chamber; (b) Lower section known as the Digestion chamber and (c) Gas vent and Scum section.
• Solids settle out in the upper Sedimentation chamber and gradually flow into the Lower digestion chamber.
• In the Lower digestion chamber, Sludge is digested Anaerobically within 2 to 4 hours.
• Scum and gas vent chambers are located at the sides of the tank.
• By design, Gas and Scum are prevented from entering the Sedimentation chamber due to the narrow slots that disallow Gas and Sludge particles from entering the Sedimentation chamber.
• Methane gas is produced in the process and is prevented from disturbing the settling process by being deflected by baffles into the gas vent channels.
• Effluent is odourless because the suspended and dissolved solids in the effluent do not come into contact with the active sludge causing it to become foul.
• When sludge is removed needs to be further treated in drying beds or such for pathogen control.

Performance of Imhoff’s tank in Pollution reduction

• COD reduction – 25 to 50 %
• BOD reduction – 20 to 40 %
• Suspended solids reduction – 50 to 70 %
• Microbial reduction - Effluent still contains pathogenic Bacteria, Cysts and Worm eggs.

Advantages of Imhoff’s tank

• Low operating cost
• Solid-liquid separation and Sludge stabilization are combined in one single unit.
• Resistant against organic shock loads.
• The effluent is not septic (with low odour).
• Small space requirement.
• Standardized design available.
• Well suited for flows less than 250,000 gallons per day.
• Simple operation and maintenance.
• No electrical requirement.
• Construction materials easily available.

Disadvantages of Imhoff’s tank

• Need skilled contractor for construction.
• Imhoff’s tank are 9 m deep and its construction is costly.
• Low reduction of pathogens and chance of contamination.
• Requires Desludging.
• Odour occurs from escaping gases.
• Effluent, sludge and scum require further treatment.
• Sludge and scum must be checked and removed.
• More complex than septic tank (but also more efficient)